1. Field of the Invention
This invention relates to a thermally responsive switch of the hermetic type in which no current flows into a thermally responsive element such as a bimetal and which is substantially responsive to an ambient temperature.
2. Description of the Prior Art
Various types of protective switches have conventionally been used to protect motors for air conditioners and refrigerators against an overheat or overcurrent in abnormal conditions. In the conventional protective switches, an operating current of the motor is caused to flow into the thermally responsive element so that the element is responsive to an overheated condition in a compressor or an overcurrent in the motor. The thermally responsive element reverses its curvature to thereby cut off the operating current when the ambient temperature or the current value is abnormally increased.
Inverter-controlled motors have recently been used widely since these motors are readily and efficiently controlled minutely. However, the inverter-controlled motors cannot sufficiently be protected by the conventional protective switches for the following reason. An operating current is normally maintained in a predetermined range in conventional motors. The protective switch is adjusted not to operate in response to the motor operating current value and to operate in response to an overcurrent resulting from an abnormal condition such as a locked rotor condition. On the other hand, the operating current is varied according to a load even in the normal operation in the inverter-controlled motors. Accordingly, there is a possibility of deficiency of accuracy in a protecting characteristic when the protective switch provided for the inverter-controlled motor is designed to execute a protecting operation by utilizing the heating of the thermally responsive element subject to the motor operating current as in the conventional protective switches. In view of this problem, the protective switch for the inverter-controlled motor needs to be a thermally responsive switch of the type in which the motor operating current is not substantially caused to flow into the thermally responsive element.
Various types of thermally responsive switches have been proposed as the above-described type for the inverter-controlled motors. In many of the proposed thermally responsive switches, however, a relatively large base made of an insulating material such as resin is required for a thermally responsive element for driving a movable contact to be disposed thereon. The base prevents the thermally responsive switch from being rendered small in size.
Japanese Examined Patent Publication No. 56-8456 (1981) discloses a thermometal cutout as an example of the proposed thermally responsive switches as described above. In the disclosed thermometal cut-out, a bimetal is provided for engaging and disengaging a movable contact with and from a fixed contact, the movable contact being carried on a movable contact support plate. The thermometal cutout is arranged so that no current flows directly into the bimetal. The bimetal has a central insertion hole into which a support protrusion provided on the insulating base is fitted. The bimetal as a metal plate requires a relatively large insulator such as the insulating base for holding it in an insulated state.
A large insulator should not be enclosed in a casing of the thermally responsive switch when the casing is formed into a hermetic one. When the insulator is made of a resin, there is a possibility that the resin produces some gas in a long period of use of the thermally responsive switch. The gas would vary the composition of a filler gas filling the interior of the casing or cause chemical changes on surfaces of the contacts such that the conductivity between the contacts would be damaged. The above-mentioned filler gas is selected in view of the thermal conductivity etc. Furthermore, when ceramics are used as the insulator, there is a less possibility of production of a gas. However, it is difficult to fix the ceramic insulator to a metal component. In both cases of the resin and the ceramics, the heat capacity of the insulator is increased such that the responsibility of the thermally responsive switch is reduced.
Japanese Unexamined Patent Publication No. 1-302628 (1989) discloses a switch device having a reduced quantity of the insulator. In the disclosed switch device, a bimetal thermally responsive switch is enclosed in a metal accommodating section or casing. One end of a bimetal plate is fixed to a fixing portion of a movable contact support plate. The movable contact support plate is driven when the bimetal plate reverses its curvature in response to a predetermined temperature. However, an insulating plate reduces an assembling efficiency since it is interposed between a conductive portion such as a fixed contact and the metal casing. Furthermore, the metal casing is attached closely to the resin base and accordingly, the switch device is not formed into a hermetic structure. This Japanese publication shows a prior art construction in which an opening of the casing is closed by a resin so that a hermetic switch is provided. This rather simple hermetic structure cannot maintain a fixed composition of the filler gas for a long period.
Japanese Examined Patent Publication No. 2-21088 (1990) discloses a thermal protector with a completely hermetic structure. In the disclosed protector, a glass casing encloses a fixed electrode and a movable electrode both disposed in an opposite relation. The movable electrode is driven by a thermally responsive element. The thermally responsive element is welded at its movable end side to a metal elastic strip together with the contact. The other end of the thermally responsive element is supported when the element reverses its curvature. Upon the reversing of the thermally responsive element, its side carrying the movable contact is driven against a biasing force of the metal elastic strip, whereby the movable and fixed contacts are disengaged. Since the thermally responsive element is welded to the metal elastic strip in this thermal protector, reversing and returning temperatures of the thermally responsive element before the assembly thereof differ from those after the assembly. Furthermore, the glass casing is used so that a high level of gas tightness is achieved. However, when this thermal protector is used in an enclosed housing for an enclosed motor-driven compressor, the glass casing susceptible to breakage requires a special attention to the handling thereof.
Furthermore, the thermally responsive element is cantilevered in each of the above-described switch device and thermal protector. In this construction, stress of the thermally responsive element is increased when the contact pressure is increased. Consequently, application of the contact pressure varies the reversing and returning temperatures set in the state of a single unit of the thermally responsive element.
Japanese Unexamined Patent Publication No. 55-148331 (1980) discloses a thermal protection switch comprising a bimetal snap plate serving as a thermally responsive element and having a central through hole. A movable contact is inserted through the central hole of the bimetal snap plate and then fixed to a spring snap plate. Upon reverse of the bimetal snap plate, the movable contact is driven against a biasing force of the spring snap plate. Since the movable contact is disposed at the center of the thermally responsive element, a force resulting from the reverse is dispersed to a peripheral portion of the thermally responsive element. Consequently, the stress of the thermally responsive element can be reduced in this construction as compared with the cantilevered thermally responsive element.
In the above-described thermal protection switch, however, a casing body and a casing cover are fixed together by crimping with an insulator being interposed therebetween. This construction quite differs from the gas tight structure intended by the present invention. Furthermore, the inside of the casing cover serves as a fixed contact in the above-described thermal protection switch. Accordingly, even a slight deformation of the casing varies a force applied to the bimetal snap plate, resulting in variations in the reversing and returning temperatures of the bimetal snap plate.
As obvious from the foregoing, the conventionally proposed thermally responsive switches of the type in which no current flows into the thermally responsive element have found unsuitable for the use in the high-pressure atmosphere, for example, in the hermetic housing of the enclosed motor-driven compressor.